Gallium nitride substrate

ABSTRACT

A gallium nitride substrate has a surface with a diameter of not less than 100 mm, a difference being not less than 0.1 cm −1  and not more than 2 cm −1  between maximum and minimum values of wave numbers at a maximum peak of peaks corresponding to an E 2   H  phonon mode in micro-Raman scattering mapping measurement at each of square regions having sides each having a length of 2 mm, the square regions being located at a total of five locations including a central location and four circumferential edge locations on the surface of the gallium nitride substrate, a difference being not more than 2 cm −1  between maximum and minimum values of the wave numbers at the maximum peak of the peaks corresponding to the E 2   H  phonon mode at all of measurement points in the five locations.

TECHNICAL FIELD

The present invention relates to a gallium nitride (GaN) substrate.

BACKGROUND ART

Among nitride semiconductor substrates, GaN substrates have been drawingattention as substrates for manufacturing semiconductor devices such asa light emitting device and an electronic device. However, at present,for manufacturing of a GaN substrate, growth has to be performed on aheterogeneous substrate. Since lattice constant and thermal expansioncoefficient are different between the heterogeneous substrate and theGaN crystal, a multiplicity of crystal defects are generated in the GaNcrystal, disadvantageously.

To address this, for example, Non-Patent Document 1 discloses a GaNsubstrate, wherein a GaN crystal provided with a multiplicity ofdot-shaped depressions in its surface is grown such that crystal defectsare located intensively at the centers of the depressions in the GaNcrystal and crystal defects are reduced around the depressions.

CITATION LIST Non Patent Document

NPD 1. Kensaku Motoki, “Development of GaN Substrates”, SEI technicalreview, Vol. 175, July, 2009, pp. 10-18

NPD 2: Hiroshi Harima, “Characterization of GaN and Related Nitrides byRaman Scattering”, Journal of the Society of Material Science, Japan,Vol. 51, No. 9, September, 2002, pp. 983-988

SUMMARY OF INVENTION Technical Problem

However, improvement has been required because such a GaN substrate maybe cracked or broken when manufacturing a semiconductor device byepitaxially growing another semiconductor layer on the GaN substrate.

Solution to Problem

A GaN substrate according to one embodiment of the present invention isa GaN substrate having a surface with a diameter of not less than 100mm, a difference being not less than 0.1 cm⁻¹ and not more than 2 cm⁻¹between maximum and minimum values of wave numbers at a maximum peak ofpeaks corresponding to an E₂ ^(H) phonon mode in micro-Raman scatteringmapping measurement at each of square regions having sides each having alength of 2 mm, the square regions being located at a total of fivelocations including a central location and four circumferential edgelocations on the surface of the GaN substrate, a difference being notmore than 2 cm⁻¹ between maximum and minimum values of the wave numbersat the maximum peak of the peaks corresponding to the E₂ ^(H) phononmode at all of measurement points in the five locations.

A bonded substrate according to one embodiment of the present inventionis a bonded substrate in which the GaN substrate is bonded to asupporting substrate.

Advantageous Effects of Invention

According to the description above, occurrence of crack and breakage canbe suppressed when epitaxially growing another semiconductor layer.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic perspective view of a GaN substrate of a firstembodiment.

Each of FIG. 2(a) to FIG. 2(d) is a schematic cross sectional viewillustrating an exemplary method for manufacturing the GaN substrate ofthe first embodiment, and FIG. 2(e) is a schematic cross sectional viewof an exemplary bonded substrate of the first embodiment.

FIG. 3 is a schematic plan view of an exemplary entire surface of theGaN substrate of the first embodiment.

FIG. 4 shows a crystal structure of a wurtzite type GaN crystal.

FIG. 5 illustrates an E₂ ^(H) phonon mode.

FIG. 6 is a conceptual view of a relation between strain and a locationon a straight line passing through points B, A, and D of the GaNsubstrate of the first embodiment.

FIG. 7 is a conceptual view of a relation between strain and a locationon a straight line passing through points B, A, and D of a conventionalGaN substrate.

FIG. 8 shows a result of micro-Raman scattering measurement of a GaNsubstrate of an experiment example 6.

DESCRIPTION OF EMBODIMENTS

[Description of Embodiments of the Present Invention]

First, embodiments of the present invention are listed and described.

(1) A GaN substrate according to one embodiment of the present inventionis a GaN substrate having a surface with a diameter of not less than 100mm, a difference being not less than 0.1 cm⁻¹ and not more than 2 cm⁻¹between maximum and minimum values of wave numbers at a maximum peak ofpeaks corresponding to an E₂ ^(H) phonon mode in micro-Raman scatteringmapping measurement at each of square regions having sides each having alength of 2 mm, the square regions being located at a total of fivelocations including a central location and four circumferential edgelocations on the surface of the GaN substrate, a difference being notmore than 2 cm⁻¹ between maximum and minimum values of the wave numbersat the maximum peak of the peaks corresponding to the E₂ ^(H) phononmode at all of measurement points in the five locations. With such aconfiguration, occurrence of crack and breakage can be suppressed whenepitaxially growing another semiconductor layer on the GaN substratehaving the surface with a diameter of not less than 100 mm.

(2) Preferably in the GaN substrate according to one embodiment of thepresent invention, the diameter is not less than 150 mm, the differenceis not less than 0.1 cm⁻¹ and not more than 1 cm⁻¹ between the maximumand minimum values of the wave numbers at the maximum peak of the peakscorresponding to the E₂ ^(H) phonon mode in the micro-Raman scatteringmapping measurement at each of the square regions having the sides eachhaving a length of 2 mm, the square regions being located at the totalof five locations including the central location and the fourcircumferential edge locations on the surface of the GaN substrate, andthe difference is not more than 1 cm⁻¹ between the maximum and minimumvalues of the wave numbers at the maximum peak of the peakscorresponding to the E₂ ^(H) phonon mode at all of the measurementpoints in the five locations. With such a configuration, occurrence ofcrack and breakage can be suppressed when epitaxially growing anothersemiconductor layer on the GaN substrate having the surface with adiameter of not less than 150 mm.

(3) Preferably in the GaN substrate according to one embodiment of thepresent invention, each of the square regions having the sides eachhaving a length of 2 mm on the surface of the GaN substrate includes: aregion having a threading dislocation density of not less than 1×10⁶cm⁻²; and a region having a threading dislocation density of less than1×10⁶ cm⁻². In this case, dislocations are gathered in the region havinga threading dislocation density of not less than 1×10⁶ cm⁻², therebyimproving crystallinity in the region having a threading dislocationdensity of less than 1×10⁶ cm⁻².

(4) Preferably in the GaN substrate according to one embodiment of thepresent invention, each of the square regions having the sides eachhaving a length of 2 mm on the surface of the gallium nitride substrateincludes: a region having an oxygen concentration of not less than5×10¹⁷ cm⁻³; and a region having an oxygen concentration of less than5×10¹⁷ cm⁻³.

(5) The GaN substrate according to one embodiment of the presentinvention is a bonded substrate in which the above-described GaNsubstrate is bonded to a supporting substrate. With such aconfiguration, occurrence of crack and breakage can be suppressed whenepitaxially growing another semiconductor layer.

[Details of Embodiments of the Present Invention]

Hereinafter, the embodiments will be described. It should be noted thatthe same reference characters indicate the same or equivalent portionsin the figures used for description of the embodiments.

[First Embodiment]

<GaN Substrate>

FIG. 1 shows a schematic perspective view of a portion of a surface of aGaN substrate of a first embodiment. A GaN substrate 10 of the firstembodiment is composed of a GaN crystal 11. Regions having threadingdislocations 23 intensively located therein are formed to extend from asurface of GaN crystal 11 toward inside of GaN crystal 11.

<Method for Manufacturing GaN Substrate>

With reference to schematic cross sectional views of FIG. 2(a) to FIG.2(d), the following describes an exemplary method for manufacturing theGaN substrate of the first embodiment. First, as shown in FIG. 2(a), agrowth substrate 21 is prepared which has a surface 21 a to serve as agrowth surface. Growth substrate 21 is not particularly limited as longas GaN crystal 11 can be grown on surface 21 a. Examples of growthsubstrate 21 may include: a heterogeneous substrate such as galliumarsenide (GaAs); and a homogeneous substrate composed of GaN.

Next, as shown in FIG. 2(b), a patterning layer 22 is formed on asurface 21 a of growth substrate 21. Patterning layer 22 can be formedby: forming a silicon oxide

(SiO₂) film through plasma CVD (Chemical Vapor Deposition) on the entiresurface 21 a of growth substrate 21; forming a resist patterned throughphotolithography on the SiO₂ film; and performing etching using theresist as an etching mask, for example.

Next, as shown in FIG. 2(c), GaN crystal 11 is grown on surface 21 a ofgrowth substrate 21 having patterning layer 22 formed thereon. GaNcrystal 11 can be grown through HVPE (Hydride Vapor Phase Epitaxy) inwhich metallic Ga is used as a gallium (Ga) source material and ammonia(NH₃) gas is used as a nitrogen (N) source material, for example.

Next, as shown in FIG. 2(d), growth substrate 21 on the backside of GaNcrystal 11 is removed by grinding or the like, for example. Then, forexample, the surface of GaN crystal 11 is planarized by grinding or thelike, and then is polished, thus obtaining GaN substrate 10 of the firstembodiment.

Moreover, for example, as shown in a schematic cross sectional view ofFIG. 2(e), a heterogeneous substrate 24 can be bonded onto the surfaceof the obtained GaN substrate 10 of the first embodiment from whichgrowth substrate 21 has been removed, thereby producing a bondedsubstrate 25. Examples of heterogeneous substrate 24 can include asapphire substrate, an AlN substrate, a SiC substrate, a GaAs substrate,a ZrB₂ substrate, a SiO₂/Al₂O₃ sintered compact substrate, a Mosubstrate, and the like.

Moreover, the method for bonding GaN substrate 10 of the firstembodiment to heterogeneous substrate 24 is not particularly limited;however, it is preferable to use a surface activation method or a fusionbonding method in order to bond them together uniformly at a lowtemperature, for example. Here, the surface activation method refers toa method for bonding them together after exposing the bonding surface ofGaN substrate 10 to plasma to activate the bonding surface, whereas thefusion bonding method refers to a method for bonding them together byheating respective washed surfaces (bonding surfaces) under a pressure.Moreover, GaN substrate 10 of the first embodiment can be bonded toheterogeneous substrate 24 with a joining film interposed therebetween.

<Δkp (2 mm□) and Δkp (Entire Surface)>

FIG. 3 shows a schematic plan view of an exemplary entire surface of GaNsubstrate 10 of the first embodiment. The surface of GaN substrate 10 ofthe first embodiment has a diameter R of not less than 100 mm. DiameterR of the surface of GaN substrate 10 refers to the diameter of animaginary circle assuming that no orientation flat 30 is formed in GaNsubstrate 10 even though orientation flat 30 is formed in GaN substrate10.

Moreover, a difference (Δkp (2 mm□)) is not less than 0.1 cm⁻¹ and notmore than 2 cm⁻¹ between maximum and minimum values of wave numbers at amaximum peak of peaks corresponding to an E₂ ^(H) phonon mode in Ramanspectra obtained through micro-Raman scattering mapping measurement in a2 mm□ plane, i.e., in each of square regions (regions 31 a, 31 b, 31 c,31 d, 31 e) having sides each having a length of 2 mm, the squareregions having respective centers located at a total of five pointsincluding a central point A and circumferential edge points B, C, D, andE in GaN substrate 10.

Further, a difference (Δkp (entire surface)) is not more than 2 cm⁻¹between maximum and minimum values of wave numbers at the maximum peakof the peaks corresponding to the E₂ ^(H) phonon mode in the Ramanspectra at all of measurement points in regions 31 a, 31 b, 31 c, 31 d,31 e.

<Methods for Determining Δkp (2 mm□) and Δkp (Entire Surface)>

The following describes methods for determining Δkp (2 mm□) and Δkp(entire surface).

«Specifying Measurement Regions»

First, regions 31 a, 31 b, 31 c, 31 d, 31 e described above arespecified as follows. Central point A on the surface of GaN substrate 10is specified as the point of center of the circle of the surface of GaNsubstrate 10 (center of the imaginary circle assuming that noorientation flat 30 is formed when orientation flat 30 is formed in GaNsubstrate 10). Then, assuming that point A thus specified is anintersection of diagonal lines, region 31 a is defined as a squareregion having: two sides constituted of two line segments having alength of 2 mm and parallel to orientation flat 30 shown in FIG. 3; andtwo sides constituted of two line segments having a length of 2 mm andperpendicular to orientation flat 30.

On the other hand, circumferential edge points B, C, D, and E on thesurface of GaN substrate 10 are specified as points on the circumferenceof an imaginary circle 32 inwardly of, by 5 mm, the outer circumferenceof the circle (imaginary circle assuming that no orientation flat 30 isformed when orientation flat 30 is formed in GaN substrate 10)constituting of the outer circumference of the surface of GaN substrate10. Circumferential edge points B, C, D, and E are in such a relationthat points B, A, and D are on one straight line, points C, A, and E areon one straight line, and the straight line connecting points B, A, D toone another is orthogonal to the straight line connecting points C, A, Eto one another. Then, assuming that each of points B, C, D, and E is anintersection of diagonal lines, each of regions 31 b, 31 c, 31 d, 31 eis defined as a square region having: two sides constituted of linesegments having a length of 2 mm, parallel to orientation flat 30 andparallel to each other; and two sides constituted of two line segmentshaving a length of 2 mm, perpendicular to orientation flat 30, andparallel to each other.

«Determination of Δkp (2 mm□)»

Next, micro-Raman scattering mapping measurement is performed at aplurality of locations in region 31 a specified as above to measureRaman spectra at the respective locations within the 2 mm□ plane ofregion 31 a, thereby specifying peaks corresponding to the E₂ ^(H)phonon mode at the locations within the 2 mm□ plane of region 31 a.Accordingly, the values of the wave numbers (corresponding to Ramanshift amounts in cm⁻¹) at the maximum peak of the peaks are specified atthe locations. Then, maximum value (a1) and minimum value (a2) arespecified from the wave numbers specified at the locations within the 2mm□ plane. Then, a difference (a1−a2) is determined between the maximumvalue (a1) and the minimum value (a2) of the wave numbers specified asdescribed above, thereby determining Δkp (2 mm□) within the 2 mm□ planeof region 31 a.

Δkp (2 mm□) of region 31 b can be also determined in a manner similar tothe determination of Δkp (2 mm□) of region 31 a, i.e., can be determinedby: performing micro-Raman scattering mapping measurement to measureRaman spectra at a plurality of locations in region 31 b; specifyingvalues of wave numbers at the maximum peak of the peaks corresponding tothe E₂ ^(H) phonon mode; and determining a difference (b1−b2) betweenmaximum value (b1) and minimum value (b2) of the wave numbers asspecified from the values of the wave numbers.

Δkp (2 mm□) of region 31 c can be also determined in a manner similar tothe determination of Δkp (2 mm□) of region 31 a, i.e., can be determinedby: performing micro-Raman scattering mapping measurement to measureRaman spectra at a plurality of locations in region 31 c; specifyingvalues of wave numbers at the maximum peak of the peaks corresponding tothe E₂ ^(H) phonon mode; and determining a difference (c1−c2) betweenmaximum value (c1) and minimum value (c2) of the wave numbers asspecified from the values of the wave numbers.

Δkp (2 mm□) of region 31 d can be also determined in a manner similar tothe determination of Δkp (2 mm□) of region 31 a, i.e., can be determinedby performing micro-Raman scattering mapping measurement to measureRaman spectra at a plurality of locations in region 31 d; specifyingvalues of wave numbers at the maximum peak of the peaks corresponding tothe E₂ ^(H) phonon mode; and determining a difference (d1−d2) betweenmaximum value (d1) and minimum value (d2) of the wave numbers asspecified from the values of the wave numbers.

Δkp (2 mm□) of region 31 e can be also determined in a manner similar tothe determination of Δkp (2 mm□) of region 31 a, i.e., can be determinedby: performing micro-Raman scattering mapping measurement to measureRaman spectra at a plurality of locations in region 31 e; specifyingvalues of wave numbers at the maximum peak of the peaks corresponding tothe E₂ ^(H) phonon mode; and determining a difference (e1−e2) betweenmaximum value (e1) and minimum value (e2) of the wave numbers asspecified from the values of the wave numbers.

<<Determination of Δkp (Entire Surface)>>

Next, the maximum value (X1) of the wave numbers is specified frommaximum values a1, b1, c1, d1, e1 of the wave numbers in regions 31 a,31 b, 31 c, 31 d, 31 e. Likewise, the minimum value (X2) of the wavenumbers is specified from minimum values a2, b2, c2, d2, e2 of the wavenumbers in regions 31 a, 31 b, 31 c, 31 d, 31 e. Then, a difference(X1−X2) between the maximum value (X1) of the wave numbers and theminimum value (X2) of the wave numbers is determined, therebydetermining Δkp (entire surface).

<E₂ ^(H) Phonon Mode>

Regarding the E₂ ^(H) phonon mode, the following describes a wurtzitetype GaN crystal as an example. The E₂ ^(H) phonon mode is such a modethat N atoms are displaced in a C plane as shown in FIG. 5 in a GaNcrystal having a crystal structure including Ga atoms (white dots) and Natoms (black dots) shown in FIG. 4.

Moreover, as described above, a Raman shift amount corresponding to theE₂ ^(H) phonon mode is specified in accordance with a wave number at themaximum peak of peaks corresponding to the E₂ ^(H) phonon mode in aRaman shift spectrum obtained through micro-Raman scattering mappingmeasurement. It should be noted that Table II on page 985 of Non-PatentDocument 2 illustrates 567.6 cm⁻¹ as a wave number for the E₂ ^(H)phonon mode in a wurtzite type GaN crystal at a temperature of 300 K. ARaman spectrum diagram of FIG. 3 of Non-Patent Document 2 shows that awave number at the maximum peak of peaks corresponding to the E₂ ^(H)phonon mode is around 567.6 cm⁻¹.

<Function and Effect>

As described above, the E₂ ^(H) phonon mode is scattered light inrelation with such a mode that adjacent N atoms in the GaN crystaloscillate in an in-plane direction within a C plane (see Non-PatentDocument 2). When compressive strain is generated in the C plane, thephonon frequency of the oscillation becomes high, resulting in a highwave number at the maximum peak of the peaks corresponding to the E₂^(H) phonon mode. On the other hand, when tensile strain is generated inthe C plane, the phonon frequency of the oscillation becomes low,resulting in a low wave number at the maximum peak of the peakscorresponding to the E₂ ^(H) phonon mode. When the compressive strain orthe tensile strain becomes too large locally (microscopically) orentirely (macroscopically) in the GaN substrate, the GaN substrate islikely to be cracked or broken when epitaxially growing anothersemiconductor layer on the GaN substrate. Hence, it is more preferablethat the wave number (corresponding to the Raman shift amount (magnitudeof strain)) at the maximum peak of the peaks corresponding to the E₂^(H) phonon mode in the GaN substrate is smaller both microscopicallyand macroscopically.

In GaN substrate 10 of the first embodiment, the difference (Δkp (2mm□)) is not less than 0.1 cm⁻¹ and not more than 2 cm⁻¹ between themaximum and minimum values of the Raman shift amounts corresponding tothe E₂ ^(H) phonon mode in the Raman spectra obtained through themicro-Raman scattering mapping measurement in each of square regions 31a, 31 b, 31 c, 31 d, 31 e having sides each having a length of 2 mm, thesquare regions having respective centers at a total of five pointsincluding central point A and circumferential edge points B, C, D and Eon the surface of GaN substrate 10. Accordingly, microscopic strain inGaN substrate 10 of the first embodiment can be reduced, thus attaininga more uniform microscopic strain distribution. It should be noted thatit is more preferable that the value of Δkp (2 mm□) in each of regions31 a, 31 b, 31 c, 31 d, 31 e is smaller. For example, in the case ofbelow-mentioned coreless growth rather than facet growth, microscopicstrain distribution becomes small but macroscopic strain distributionbecomes large, with the result that GaN substrate 10 is likely to bebroken or cracked during epitaxial growth of a semiconductor layer onGaN substrate 10. To address this, in GaN substrate 10 of the firstembodiment, a microscopic strain distribution as in the facet growth isintentionally produced to suppress increased macroscopic straindistribution, thereby suppressing occurrence of breakage or crack in GaNsubstrate 10 during epitaxial growth of a semiconductor layer on GaNsubstrate 10. In order to produce the microscopic strain distribution,Δkp (2 mm□) preferably has a value not less than a certain value, and ispreferably set at not less than 0.1 cm⁻¹.

Moreover, in GaN substrate 10 of the first embodiment, the difference(Δkp (entire surface)) is not more than 2 cm⁻¹ between the maximum andminimum values of the Raman shift amounts corresponding to the E₂ ^(H)phonon mode in the Raman spectra at all of the measurement points inregions 31 a, 31 b, 31 c, 31 d, 31 e. In this way, the macroscopicstrain can be reduced in GaN substrate 10 of the first embodiment,thereby attaining a more uniform macroscopic strain distribution.

This is achieved because it has been found, as a result of diligentstudy by the present inventor, that even when epitaxially growinganother semiconductor layer on a GaN substrate having a large-diametersurface having a diameter R of not less than 100 mm, the GaN substratecan be suppressed from being cracked or broken if Δkp (2 mm□) and Δkp(entire surface) are set as described above.

FIG. 6 shows a conceptual view of a relation between strain and alocation on the straight line passing through points B, A, and D of GaNsubstrate 10 of the first embodiment. For comparison, FIG. 7 shows aconceptual view of a relation between strain and a location on astraight line passing through points B, A, and D of a conventional GaNsubstrate.

Apparently from the comparison between FIG. 6 and FIG. 7, it isunderstood that in GaN substrate 10 of the first embodiment, strain isreduced to be small at any of points B, A, and D and a differencebetween the magnitude of maximum strain and the magnitude of minimumstrain is reduced to be small in the entire GaN substrate as comparedwith the conventional GaN substrate.

Although the GaN substrate employing the GaN crystal described inNon-Patent Document 1 can be suppressed from being cracked and broken,GaN substrate 10 of the first embodiment provides a more improved effectof suppressing occurrence of crack and breakage because Δkp (2 mm□) andΔkp (entire surface) are set as described above.

It should be noted that the term “crack” refers to a fissure formed inGaN substrate 10 to such an extent that GaN substrate 10 is not dividedinto plural pieces. On the other hand, the term “breakage” refers tosuch a state that GaN substrate 10 is fissured and divided into pluralpieces.

[Second Embodiment]

A feature of a GaN substrate 10 of a second embodiment lies in that itis different from the first embodiment in the following points (i) to(iii).

(i) GaN substrate 10 has a surface having a diameter R of not less than150 mm.

(ii) A difference (Δkp (2 mm□)) is not less than 0.1 cm⁻¹ and not morethan 1 cm⁻¹ between maximum and minimum values of Raman shift amountscorresponding to the E₂ ^(H) phonon mode in Raman spectra obtainedthrough micro-Raman scattering mapping measurement in each of squareregions 31 a, 31 b, 31 c, 31 d, 31 e having sides each having a lengthof 2 mm, the square regions having respective centers at a total of fivepoints including a central point A and circumferential edge points B, C,D and E on the surface of GaN substrate 10.

(iii) A difference (Δkp (entire surface)) is not more than 1 cm⁻¹between maximum and minimum values of the Raman shift amountscorresponding to the E₂ ^(H) phonon mode in the Raman spectra at all ofthe measurement points in regions 31 a, 31 b, 31 c, 31 d, 31 e.

In the second embodiment, since the lower limit of diameter R of thesurface of GaN substrate 10 is larger, i.e., the surface of GaNsubstrate 10 has a larger diameter, crack and breakage are more likelyto occur as compared with the first embodiment. However, also in thiscase, occurrence of crack and breakage can be suppressed by setting Δkp(2 mm□) at not less than 0.1 cm⁻¹ and not more than 1 cm⁻¹ and settingΔkp (entire surface) at not more than 1 cm⁻¹.

The second embodiment is the same as the first embodiment apart from theabove description, and therefore repeated description thereof will notbe provided.

[Third Embodiment]

A feature of a GaN substrate 10 of a third embodiment lies in that asquare region having sides each having a length of 2 mm on a surface ofGaN substrate 10 include: a region having a threading dislocationdensity of not less than 1×10⁶ cm⁻²; and a region having a threadingdislocation density of less than 1×10⁶ cm⁻². In GaN substrate 10 of thethird embodiment, dislocations are located intensively in a region (coreportion 12) having a threading dislocation density of not less than1×10⁶ cm⁻², thereby improving crystallinity of the region (facet 13)having a threading dislocation density of less than 1×10⁶ cm⁻².

Each square region having sides each having a length of 2 mm on thesurface of GaN substrate 10 includes the region having a threadingdislocation density of not less than 1×10⁶ cm⁻² and the region having athreading dislocation density of less than 1×10⁶ cm⁻² as in the thirdembodiment presumably because the following phenomena (I) to (V) occurin this order in the course of the crystal growth of GaN crystal 11 fromFIG. 2(b) to FIG. 2(c).

(I) Threading dislocations are reduced in facets 13 of each depression14 in the surface of GaN crystal 11 because the dislocations are movedto a boundary between adjacent facets 13.

(II) A defect surface (surface defect portion) is formed because thedislocations are gathered below the boundary between adjacent facets 13of depression 14 in the surface of GaN crystal 11.

(III) The dislocations are prevented from being expanded by merging andconfinement of the dislocations at multiple points at which theplurality of facets 13 of depressions 14 in the surface of GaN crystalII cross with one another.

(IV) Line defect portions and core portions 12 above the line defectportions are formed due to the dislocations gathering below the multiplepoints.

(V) The low defect portions are increased in facets 13 due to growth offacets 13.

The third embodiment is the same as the first embodiment and the secondembodiment apart from the above descriptions, and therefore repeateddescription thereof will not be provided. In other words, GaN substrate10 of the third embodiment includes not only the region having athreading dislocation density of not less than 1×10⁶ cm⁻² and the regionhaving a threading dislocation density of less than 1×10⁶ cm⁻², but alsothe features of GaN substrate 10 of the first embodiment or the secondembodiment.

It should be noted that the threading dislocations are dislocationsthreading in the C axis growth direction, and the threading dislocationdensity can be evaluated by counting a density of etch pits resultingfrom selective etching. Examples of the selective etching methodinclude: immersion of the GaN substrate in a heated acid or alkalineaqueous solution; immersion of the GaN substrate in a molten salt ofpotassium hydroxide (molten KOH); or the like. Moreover, the threadingdislocation density can be also measured using cathode luminescence(CL). In the CL, portions with threading dislocations are shown as darkpoints. Hence, the number of the dark points is counted to determine adensity per unit area (1 cm²), thereby measuring the threadingdislocation density.

EXAMPLES Experiment Example 1

First, as shown in FIG. 2(a), as growth substrate 21, there was prepareda sapphire substrate having a surface (C plane) 21 a having a diameterof 110 mm. Next, as shown in FIG. 2(b), a SiO₂ film having a thicknessof 0.1 μm was formed on the C plane of the sapphire substrate throughplasma CVD, and then photolithography and etching employing BHF(buffered hydrofluoric acid) were performed to form a patterning layer22 constituted of a SiO₂ film. Patterning layer 22 had such a shape thatcircles each having a diameter of 50 μm were arranged in the form of alattice at a pitch of 800 μm, and lattice directions were matched withan m-axis direction and an a-axis direction.

Next, as shown in FIG. 2(c), GaN crystal 11 was grown for 10 hours tohave a thickness of about 1200 μm, on the C plane of the sapphiresubstrate having patterning layer 22 formed thereon and serving asgrowth substrate 21. GaN crystal 11 was grown through HVPE employingmetallic Ga as a Ga source material and employing NH₃ gas as a N sourcematerial.

GaN crystal 11 was grown through HVPE as follows. First, the sapphiresubstrate serving as growth substrate 21 was placed on a sample holdermade of quartz in a hot wall type reactor. Hydrogen chloride (HCl) gaswas sprayed to metallic Ga (heated at 800° C.) placed in anupstream-side boat while using hydrogen (H₂) gas as a carrier gas.Resulting gallium chloride (GaCl) gas and NH₃ gas were supplied for 30minutes onto the sapphire substrate heated at 500° C., thereby forming alow-temperature GaN buffer layer having a thickness of about 50 nm.Then, the sapphire substrate was heated to attain a temperature of 1000°C. at the center of the sapphire substrate, and GaCl gas (3.06 kPa) andNH₃ gas (6.12 kPa) were supplied to the sapphire substrate for 10 hourstogether with H₂ gas serving as a carrier gas. In this way, GaN crystal11 having a thickness of about 1200 μm was grown. Here, a temperaturedifference ΔT in the radial direction of the sapphire substrate(temperature difference between the temperature of the center and thetemperature of the circumferential edge (location away from the centerby a radius of 55 mm)) was 2° C.

Then, the sapphire substrate was removed by grinding the backsidesurface of GaN crystal 11 having been grown as described above. Next,the surface of GaN crystal 11 was planarized through grinding and wasthen polished, thereby producing a GaN substrate of experiment example 1(with a finishing thickness of 500 μm), which was a freestanding GaNsubstrate having a circular shape with a diameter of 100 mm, having theC plane as a surface, and having a facet structure.

Next, micro-Raman mapping measurement was performed as follows. As alight source, a second harmonic wave laser device with YAG (yttriumaluminum garnet) was used. Laser light having a wavelength of 532 nm wasemitted from the laser device, passed through a slit having a width of100 μm, and was collected by a lens, whereby the light was incidentperpendicularly from the front surface side (side opposite to the sidefrom which the sapphire substrate had been removed) of the GaN substrateof experiment example 1.

Here, the laser light was set to have a spot diameter of about 10 μm onthe surface of the GaN substrate. Moreover, the laser light was set tohave an intensity of 10 mW on the surface of the GaN substrate. Then,micro-Raman scattering mapping measurement for detecting lightback-scattered in the C axis direction was performed to measure Ramanspectra. The Raman spectra were measured at points (1681 points in eachof the regions) located at a pitch of 50 μm in each of square regions 31a, 31 b, 31 c, 31 d, 31 e having sides each having a length of 2 mm, thesquare regions having respective centers at central point A andcircumferential points B, C, D, and E in the GaN substrate shown in FIG.3.

Then, a difference (Δkp (2 mm□)) was determined between maximum andminimum values of wave numbers at the maximum peak of peakscorresponding to the E₂ ^(H) phonon mode in the Raman spectra obtainedthrough the micro-Raman scattering mapping measurement in each of squareregions 31 a, 31 b, 31 c, 31 d, 31 e having sides each having a lengthof 2 mm, the square regions having respective centers at the total offive points including central point A and outer circumferential edgepoints B, C, D and E in the GaN substrate of experiment example 1. Inaddition, a difference (Δkp (entire surface)) was determined betweenmaximum and minimum values of the wave numbers at the maximum peak ofthe peaks corresponding to the E₂ ^(H) phonon mode in the Raman spectraat all of the measurement points in regions 31 a, 31 b, 31 c, 31 d, 31e. Results thereof are shown in Table 1. It should be noted that atemperature was 20° C. when measuring the Raman spectra. For wave numbercalibration, a bright line spectrum of Ne lamp was used as a referenceline. For each measurement, Ne bright line was measured for correction.Then, in the Raman spectra obtained as described above, the wave numbersat the maximum peak of the peaks corresponding to the E₂ ^(H) phononmode was specified. Moreover, in Table 1, “kp maximum value” representsthe maximum value of the wave numbers at the maximum peak of the peakscorresponding to the E₂ ^(H) phonon mode, and “kp minimum value”represents the minimum value of the wave numbers at the maximum peak ofthe peaks corresponding to the E₂ ^(H) phonon mode.

Experiment Example 2

A C-plane GaN template substrate was used which had a surface having adiameter of 110 mm and had a GaN film having a thickness of 2 μm andformed through MOCVD on a sapphire substrate serving as growth substrate21. A patterning layer 22 constituted of a SiO₂ film was formed in thesame manner as in experiment example 1. A GaN crystal was grown onpatterning layer 22 by the same method and under the same conditions asthose in experiment example 1 without forming a low-temperature GaNbuffer layer. In this way, a GaN substrate of experiment example 2 wasproduced (with a finishing thickness of 500 μm), which was afreestanding GaN substrate having a circular shape with a diameter of100 mm, having a C plane as a surface, and having a facet structure.

Then, in the same manner as in experiment example 1, a difference (Δkp(2 mm□)) was determined between maximum and minimum values of wavenumbers at the maximum peak of peaks corresponding to the E₂ ^(H) phononmode in the Raman spectra obtained through the micro-Raman scatteringmapping measurement in each of square regions 31 a, 31 b, 31 c, 31 d, 31e having sides each having a length of 2 mm, the square regions havingrespective centers at a total of five points including a central point Aand outer circumferential edge points B, C, D and E in the GaN substrateof experiment example 2. In addition, a difference (Δkp (entiresurface)) was determined between maximum and minimum values of the wavenumbers at the maximum peak of the peaks corresponding to the E₂ ^(H)phonon mode in the Raman spectra at all of the measurement points inregions 31 a, 31 b, 31 c, 31 d, 31 e. Results thereof are shown in Table1.

Experiment Example 3

With the same method and under the same conditions as those inexperiment example 1, a low-temperature GaN buffer layer is formed and aGaN crystal is grown on a GaAs substrate serving as growth substrate 21and having a surface ((111) A plane) having a diameter of 110 mm. Inthis way, a GaN substrate of experiment example 3 was produced (with afinishing thickness of 500 μm), which was a freestanding GaN substratehaving a circular shape with a diameter of 100 mm, having a C plane as asurface, and having a facet structure.

Then, in the same manner as in experiment example 1, a difference (Δkp(2 mm□)) was determined between maximum and minimum values of wavenumbers at the maximum peak of peaks corresponding to the E₂ ^(H) phononmode in the Raman spectra obtained through the micro-Raman scatteringmapping measurement in each of square regions 31 a, 31 b, 31 c, 31 d, 31e having sides each having a length of 2 mm, the square regions havingrespective centers at a total of five points including a central point Aand outer circumferential edge points B, C, D and E in the GaN substrateof experiment example 3. In addition, a difference (Δkp (entiresurface)) was determined between maximum and minimum values of the wavenumbers at the maximum peak of the peaks corresponding to the E₂ ^(H)phonon mode in the Raman spectra at all of the measurement points inregions 31 a, 31 b, 31 c, 31 d, 31 e. Results thereof are shown in Table1.

Experiment Example 4

As a substrate, a sapphire substrate having a diameter of 110 mm wasused as in experiment example 1. A GaN crystal was grown with the samemethod and under the same conditions as those in experiment example 1.In this way, a freestanding GaN substrate was produced (with a finishingthickness of 500 μm), which had a circular shape with a diameter of 100mm, had a C plane as a surface, and had a facet structure. Then, in thesame manner as in experiment example 1, Δkp (2 mm□) and Δkp (entiresurface) were determined. Results thereof are shown in Table 1.

It should be noted that at an early stage of growth of the GaN crystal,particularly, for 10 minutes after starting the crystal growth, anamount of oxygen included in atmosphere in the crystal growth furnacewas set at not more than 100 ppm. Specifically, before starting thegrowth of the GaN crystal, gases such as N₂, H₂ and Ar were supplied ata room temperature for not less than 10 minutes to replace theatmosphere in the crystal growth furnace and an oxygen concentration inthe crystal growth furnace was monitored using an oxygen concentrationdetector to set the oxygen concentration at not more than 100 ppm. Alsoafter starting the growth of the GaN crystal, measurement and controlwere performed to set the oxygen concentration in the crystal growthfurnace at not more than 100 ppm.

Experiment Example 5

As a substrate, a C-plane GaN template substrate having a diameter of110 mm was used as in experiment example 2. A GaN crystal was grown withthe same method and under the same conditions as those in experimentexample 4 without forming a low-temperature buffer layer. In this way, afreestanding GaN substrate was produced (with a finishing thickness of500 μm), which had a circular shape with a diameter of 100 mm, had a Cplane as a surface, and had a facet structure. Then, in the same manneras in experiment example 1, Δkp (2 mm□) and Δkp (entire surface) weredetermined. Results thereof are shown in Table 1.

Experiment Example 6

A GaN crystal was grown with the same method and under the sameconditions as those in experiment example 1 except that a GaAs substratehaving a surface ((111) A plane) with a diameter of 110 mm as inexperiment example 3 was used as a substrate and oxygen concentrationcontrol was performed at an early stage of growth. In this way, afreestanding GaN substrate was produced (with a finishing thickness of500 μm), which had a circular shape with a diameter of 100 mm, had a Cplane as a surface, and had a facet structure. Then, in the same manneras in experiment example 1, Δkp (2 mm□) and Δkp (entire surface) weredetermined. Results thereof are shown in Table 1.

Experiment Example 7

A GaN crystal was grown on a GaN substrate serving as growth substrate21, with the same method and under the same conditions as those inexperiment example 5 without forming a low-temperature GaN buffer layer.The GaN substrate was produced with the same method and under the sameconditions as those in experiment example 5, and had a surface (C plane)having a diameter of 110 mm. In this way, a GaN substrate of experimentexample 7 was produced (with a finishing thickness of 500 μm), which wasa freestanding GaN substrate having a circular shape with a diameter of100 mm, having a C plane as a surface, and having a facet structure.Then, in the same manner as in experiment example 1, Δkp (2 mm□) and Δkp(entire surface) were determined. Results thereof are shown in Table 1.

<Evaluation on Epitaxial Growth>

MOVPE was employed to form a Schottky barrier diode (SBD) structurethrough epitaxial growth on each of the GaN substrates of experimentexamples 1 to 7 produced as described above. For the SBD structure, ann⁺ GaN layer and an n⁻ GaN layer were grown epitaxially in this order.The n⁺ GaN layer served as a carrier stop layer, had a carrierconcentration of 2×10¹⁸ cm⁻³, and had a thickness of 1 μm. The n⁻ GaNlayer served as a carrier drift layer, had a carrier concentration of1×10¹⁶ cm⁻³, and had a thickness of 5 μm. Epitaxial growth conditionsfor these layers were as follows: a growth temperature was 1050° C.; TMG(trimethylgallium) and NH₃ gas were used as source materials of GaN; andsilane (SiH₄) gas was used as a source material of silicon (Si) dopant.Then, an external appearance of the surface of each of the GaNsubstrates of experiment examples 1 to 7 after the epitaxial growth wasobserved. Results thereof are shown in Table 1.

As shown in Table 1, there were cracks in the surfaces of the GaNsubstrates of experiment examples 1 to 4 after the epitaxial growth.However, the GaN substrates of experiment examples 5 to 7 were notcracked and broken and had excellent external appearances. It should benoted that the crack herein was defined as a crack having a length ofnot less than 0.1 mm, which can be recognized by a Nomarski microscope(with a magnification of ×50).

Experiment Example 8

As a growth substrate, a GaN template substrate formed in the samemanner as in experiment example 1 was used. Without forming patterninglayer 22 constituted of a SiO₂ film and low-temperature buffer layer,heating was performed such that the temperature of the center of the GaNtemplate substrate became 1100° C. in order to grow GaN crystal 11 tohave a mirror surface, and the GaN template substrate was supplied withGaCl gas (2.40 kPa) and NH₃ gas (2.40 kPa) as well as N₂ gas serving asa carrier gas. Accordingly, GaN crystal 11 having a thickness of about 1mm was grown. Then, a process similar to that in experiment example 1was performed to produce a GaN substrate of experiment example 8 (with afinishing thickness of 500 μm), which was a freestanding GaN substratehaving a circular shape with a diameter of 100 mm, having a C plane as asurface, and having a coreless structure (structure having no depressionconstituted of a core portion and a facet).

Then, in the same manner as in experiment example 1, a difference (Δkp(2 mm□)) was determined between maximum and minimum values of wavenumbers at the maximum peak of peaks corresponding to the E₂ ^(H) phononmode in Raman spectra obtained through micro-Raman scattering mappingmeasurement in each of square regions 31 a, 31 b, 31 c, 31 d, 31 ehaving sides each having a length of 2 mm, the square regions havingrespective centers at a total of five points including a central point Aand outer circumferential edge points B, C, D and E in the GaN substrateof experiment example 8. In addition, a difference (Δkp (entiresurface)) was determined between maximum and minimum values of the wavenumbers at the maximum peak of the peaks corresponding to the E₂ ^(H)phonon mode in the Raman spectra at all of the measurement points inregions 31 a, 31 b, 31 c, 31 d, 31 e. Results thereof are shown in Table1.

A SBD structure was formed on the GaN substrate of experiment example 8through epitaxial growth in the same manner as in experiment example 1.However, when the GaN substrate of experiment example 8 was taken outafter the epitaxial growth, the GaN substrate of experiment example 8was broken into pieces. The GaN substrate of experiment example 8 wasbroken presumably due to stress generated during the epitaxial growthfor the SBD structure or during cooling after the epitaxial growth forthe SBD structure. It should be noted that experiment examples 5 to 7are the present examples, whereas experiment examples 1 to 4 and 8 arecomparative examples. Moreover, Table 2 shows manufacturing conditionsin the methods for manufacturing the GaN substrates of experimentexamples 1 to 8.

TABLE 1 Experiment Experiment Experiment Experiment ExperimentExperiment Experiment Experiment Example 1 Example 2 Example 3 Example 4Example 5 Example 6 Example 7 Example 8 Diameter [mm] 100 100 100 100100 100 100 100 Crystal Structure Facet Facet Facet Facet Facet FacetFacet Coreless Region Kp Maximum Value 570.24 569.31 568.77 569.45567.71 567.25 567.32 568.14 31a Kp Minimum Value 567.13 567.37 567.04567.13 566.12 567.02 567.21 568.07 [cm⁻¹] Δkp (2 mm□) 3.11 1.94 1.732.32 1.49 0.23 0.11 0.07 Region Kp Maximum Value 571.48 569.82 568.93569.95 567.59 567.93 567.55 571.31 31b Kp Minimum Value 567.81 567.35567.03 567.81 566.17 567.59 567.42 571.16 [cm⁻¹] Δkp (2 mm□) 3.67 2.471.9 2.14 1.42 0.34 0.13 0.15 Region Kp Maximum Value 570.73 568.89569.12 570.12 567.15 567.94 567.46 571.8 31c Kp Minimum Value 567.57567.01 567.09 567.63 565.68 567.63 567.33 571.65 [cm⁻¹] Δkp (2 mm□) 3.161.88 2.03 2.49 1.47 0.31 0.13 0.15 Region Kp Maximum Value 571.57 569.97569.27 569.81 567.52 567.67 567.61 571.52 31d Kp Minimum Value 567.76568.2 567.12 567.37 565.73 567.31 567.49 571.34 [cm⁻¹] Δkp (2 mm□) 3.811.77 2.15 2.44 1.79 0.36 0.12 0.18 Region Kp Maximum Value 570.93 570.02568.92 570.07 567.57 568.12 567.63 570.87 31e Kp Minimum Value 567.72567.87 567.16 567.62 565.78 567.76 567.51 570.71 [cm⁻¹] Δkp (2 mm□) 3.212.15 1.76 2.45 1.79 0.36 0.12 0.16 Δkp (Entire Surface) [cm⁻¹] 4.44 3.012.24 2.99 1.93 1.1 0.42 3.73 External Appearance after Crack Crack CrackCrack Excellent Excellent Excellent Breakage Epitaxial Growth

TABLE 2 Experiment Experiment Experiment Experiment ExperimentExperiment Experiment Experiment Example 1 Example 2 Example 3 Example 4Example 5 Example 6 Example 7 Example 8 Oxygen Concentration NotControlled Not Controlled Not Controlled Controlled ControlledControlled Controlled Controlled Control Sample Holder Quartz QuartzQuartz Quartz Quartz Quartz Quartz Quartz Growth Substrate Sapphire GaNGaAs Sapphire GaN GaAs GaN GaN Template Template Template DepressionPattern Dot Dot Dot Dot Dot Dot Dot None ΔT [° C.] 2 2 2 2 2 2 2 2 T [°C.] 1000 1000 1000 1000 1000 1000 1000 1100 GaCl Partial Pressure [kPa]3.06 3.06 3.06 3.06 3.06 3.06 3.06 2.40 NH₃ Partial Pressure [kPa] 6.126.12 6.12 6.12 6.12 6.12 6.12 2.40 Formation of Low- Formed Not FormedFormed Formed Not Formed Formed Not Formed Not Formed Temperature BufferLayer

<Evaluation on GaN Substrates of Experiment Examples 1 to 8>

FIG. 8 shows a result of micro-Raman spectrometry of the GaN substrateof experiment example 6. The result of the micro-Raman spectrometryshown in FIG. 8 indicates respective distributions of wave numbers atthe maximum peak of the peaks corresponding to the E₂ ^(H) phonon modeon a straight line including the core portions and a straight lineincluding no core portions, each of the core portions being a regionhaving dislocations intensively located therein in the square regionthat had sides each having a length of 2 mm and that had diagonal lineswhose intersection was central point A of the surface of the GaNsubstrate of experiment example 6. As shown in FIG. 8, a distribution inthe GaN substrate of experiment example 6 was such that strains weregreatly changed in the vicinity of the cores and strains were notgreatly changed in regions way from the cores.

As shown in FIG. 8, the wave numbers were changed to be small in thecore portions. This means that tensile strains were generated. Althoughit is unclear what caused the tensile strains in the core portions, thetensile strains were generated presumably due to strains resulting fromthe dislocations intensively located in the core portions or due tostrains resulting from differences between the facet-surface growthregion and the C-plane growth region in type and amount of includedimpurity.

Moreover, micro-Raman scattering mapping measurement was also performedfor each of circumferential edge points B, C, D, and E on the surface ofthe GaN substrate of experiment example 6. As a result, it was confirmedthat tendencies similar to that for central point A were exhibited.

From the above result, the microscopic strain distribution is consideredto be more dominant than the macroscopic strain distribution in the GaNsubstrate produced from the GaN crystal having the facet structureincluding (i) the core portions serving as the regions in which thedislocations were located intensively and (ii) the facets disposedaround the core portions and not serving as the regions in which thedislocations were located intensively.

It should be noted that the multiplicity of cracks were generated in thesurfaces of the GaN substrates of experiment examples 1 to 4 after theepitaxial growth presumably due to residual strains resulting from thefacet structure and thermal strains resulting from the epitaxial growthstep because all or part of Δkp (2 mm□) of regions 31 a, 31 b, 31 c, 31d, 31 e were more than 2 cm⁻¹ in the GaN substrates of experimentexamples 1 to 4. In each of the GaN substrates of experiment examples 5to 7, occurrence of cracks was not observed presumably because Δkp (2mm□) was relatively small.

Δkp (2 mm□) of each of the GaN substrates of experiment examples 4 to 6was relatively smaller than that of each of the GaN substrates ofexperiment examples 1 to 3 employing the similar substrates, presumablybecause the oxygen concentration in the crystal growth furnace at theearly stage of crystal growth was controlled to be low, i.e., not morethan 100 ppm. With the low oxygen concentration, it was considered thatcrystallinity of the GaN crystal at the early stage of crystal growthcould be improved to realize the facet structure with a smallmicroscopic strain distribution.

Moreover, Δkp (2 mm□) of the GaN substrate of experiment example 7 wasfurther smaller, presumably due to the control of oxygen concentrationat the early stage as well as further reduced defects resulting fromemploying the GaN substrate as the growth substrate, i.e., homoepitaxialgrowth.

Moreover, the GaN substrate of experiment example 8 having the corelessstructure was broken presumably because the microscopic strain was small(Δkp (2 mm□)=0.07-0.18 cm⁻¹) but the macroscopic strain was large (Δkp(entire surface)=3.73 cm⁻¹) in the GaN substrate. The GaN substrate ofexperiment example 8 was broken presumably because yield strain of theGaN substrate of experiment example 8 was exceeded by a total strainincluding residual strain and thermal strain in any of the series ofepitaxial growth steps of temperature increase, epitaxial growth, andtemperature decrease due to large compressive strain producedmacroscopically.

The microscopic strain distribution of the GaN substrate of experimentexample 8 was relatively uniform (Δkp (2 mm□)≤0.2 cm⁻¹) presumablybecause regions having dislocations were distributed more uniformly dueto absence of the regions in which the dislocations were intensivelylocated. Moreover, the macroscopic strain of the GaN substrate ofexperiment example 5 was large presumably due to macroscopic strainresulting from stress caused by mismatch in thermal expansioncoefficient and mismatch in lattice constant in heteroepitaxial growth.

As described above, considering both the microscopic strain and themacroscopic strain provides an index for defects during epitaxialgrowth. Quantitively, it is considered that occurrence of crack andbreakage can be suppressed during epitaxial growth when Δkp (2 mm□) ofeach of regions 31 a, 31 b, 31 c, 31 d, 31 e is not less than 0.1 cm⁻¹and not more than 2 cm⁻¹ and Δkp (entire surface) is not more than 2cm⁻¹.

Experiment Example 9

A GaAs substrate having a surface ((111) A plane) having a diameter of160 mm was used as growth substrate 21. With the same method and underthe same conditions as those in experiment example 6, a GaN substrate ofexperiment example 9 was produced (with a finishing thickness of 600μm), which was a freestanding GaN substrate having a circular shape witha diameter of 150 mm, having a C plane as a surface, and having a facetstructure. Here, a temperature difference ΔT in the radial direction(temperature difference between the temperature of the center and thetemperature of the circumferential edge (location way from the center bya radius of 75 mm)) was 6° C.

Then, in the same manner as in experiment example 1, a difference (Δkp(2 mm□)) was determined between maximum and minimum values of wavenumbers at the maximum peak of peaks corresponding to the E₂ ^(H) phononmode in Raman spectra obtained through micro-Raman scattering mappingmeasurement in each of square regions 31 a, 31 b, 31 c, 31 d, 31 ehaving sides each having a length of 2 mm, the square regions havingrespective centers at a total of five points including a central point Aand outer circumferential edge points B, C, D and E in the GaN substrateof experiment example 9. In addition, a difference (Δkp (entiresurface)) was determined between maximum and minimum values of the wavenumbers at the maximum peak of the peaks corresponding to the E₂ ^(H)phonon mode in the Raman spectra at all of the measurement points inregions 31 a, 31 b, 31 c, 31 d, 31 e. Results thereof are shown in Table3. It should be noted that circumferential points B, C, D, and E of theGaN substrate of experiment example 9 are located 5 mm away from theouter circumference of a circle constituting the outer circumference ofthe surface of the GaN substrate of experiment example 9.

All of Δkp (2 mm□) in regions 31 a, 31 b, 31 c, 31 d, 31 e of the GaNsubstrate of experiment example 9 were not more than 2 cm⁻¹; however,Δkp (entire surface) was 2.89 cm⁻¹, which was more than 2 cm⁻¹,resulting in large macroscopic strain. Accordingly, it was confirmedthat the GaN substrate of experiment example 9 was broken when the GaNsubstrate of experiment example 9 was evaluated in terms of epitaxialgrowth in the same manner as in experiment examples 1 to 8. This ispresumably because the large diameter of the GaN substrate led to anincreased maximum value of strain generated in the GaN substrate.

Experiment Example 10

It was considered that the macroscopic strain was increased in the GaNsubstrate of experiment example 9 due to a large variation intemperature distribution in the radial direction of the growth substrateduring the GaN crystal growth. To address this, the material of thesample holder was changed from quartz to silicon carbide (SiC) coatedgraphite having high thermal conductivity, and temperature difference ΔTwas set at 3° C. Apart from these, with the same method and under thesame conditions as those for experiment example 9, a GaN substrate ofexperiment example 10 was produced (with a finishing thickness of 600μm), which was a freestanding GaN substrate having a circular shape witha diameter of 150 mm, having a C plane as a surface, and having a facetstructure.

Then, in the same manner as in experiment example 9, a difference (Δkp(2 mm□)) was determined between maximum and minimum values of wavenumbers at the maximum peak of peaks corresponding to the E₂ ^(H) phononmode in the Raman spectra obtained through the micro-Raman scatteringmapping measurement in each of square regions 31 a, 31 b, 31 c, 31 d, 31e having sides each having a length of 2 mm, the square regions havingrespective centers at a total of five points including a central point Aand outer circumferential edge points B, C, D and E in the GaN substrateof experiment example 10. In addition, a difference (Δkp (entiresurface)) was determined between maximum and minimum values of the wavenumbers at the maximum peak of the peaks corresponding to the E₂ ^(H)phonon mode in the Raman spectra at all of the measurement points inregions 31 a, 31 b, 31 c, 31 d, 31 e. Results thereof are shown in Table3.

In the GaN substrate of experiment example 10, as compared with the GaNsubstrate of experiment example 9, both the microscopic strain and themacroscopic strain were small. Δkp (entire surface) of the GaN substrateof experiment example 10 was 1.5 cm⁻¹. It should be noted that themicroscopic strain was decreased presumably because thermal strainaffecting the GaN crystal was decreased due to the reduced temperaturedistribution in the radial direction.

When the GaN substrate of experiment example 10 was evaluated in termsof epitaxial growth in the same manner as in experiment examples 1 to 8,the GaN substrate of experiment example 10 was not broken but wascracked particularly at its circumferential edge portion.

Experiment Example 11

As growth substrate 21, the GaN substrate of experiment example 10 wasused which was produced with the same method and under the sameconditions as those for experiment example 10. With the same method andunder the same conditions as those for experiment example 10 except thatno low-temperature buffer layer was formed, a GaN substrate ofexperiment example 11 was produced (with a finishing thickness of 600μm), which was a freestanding GaN substrate having a circular shape witha diameter of 150 mm, having a C plane as a surface, and having a facetstructure.

Then, in the same manner as in experiment example 10, a difference (Δkp(2 mm□)) was determined between maximum and minimum values of wavenumbers at the maximum peak of peaks corresponding to the E₂ ^(H) phononmode in Raman spectra obtained through micro-Raman scattering mappingmeasurement in each of square regions 31 a, 31 b, 31 c, 31 d, 31 ehaving sides each having a length of 2 mm, the square regions havingrespective centers at a total of five points including a central point Aand outer circumferential edge points B, C, D and E in the GaN substrateof experiment example 11. In addition, a difference (Δkp (entiresurface)) was determined between maximum and minimum values of the wavenumbers at the maximum peak of the peaks corresponding to the E₂ ^(H)phonon mode in the Raman spectra at all of the measurement points inregions 31 a, 31 b, 31 c, 31 d, 31 e. Results thereof are shown in Table3.

In the GaN substrate of experiment example 11, as compared with the GaNsubstrate of experiment example 10, improvement was achieved with regardto the macroscopic strain. Δkp (entire surface) of the GaN substrate ofexperiment example 11 was 0.93 cm⁻¹. It should be noted that theimprovement was achieved with regard to the macroscopic strainpresumably because mechanical strain in the GaN substrate of experimentexample 11 in relation with the mismatch in thermal expansioncoefficient with that of the growth substrate could be reduced.

When the GaN substrate of experiment example 11 was also evaluated interms of epitaxial growth in the same manner, the GaN substrate ofexperiment example 11 was not cracked or broken, thus obtaining anexcellent result.

It should be noted that experiment example 11 is the present example,whereas experiment examples 9 and 10 are comparative examples. Moreover,Table 4 shows manufacturing conditions in the method for manufacturingthe GaN substrates of experiment examples 9 to 11.

TABLE 3 Experiment Experiment Experiment Example 9 Example 10 Example 11Diameter [mm] 150 150 150 Crystal Structure Facet Facet Facet Region KpMaximum Value 567.13 568.16 567.7 31a Kp Minimum Value 566.22 567.55567.47 [cm⁻¹] Δkp (2 mm□) 0.91 0.61 0.23 Region Kp Maximum Value 569.11568.92 568.21 31b Kp Minimum Value 567.88 568.29 567.76 [cm⁻¹] Δkp (2mm□) 1.23 0.63 0.45 Region Kp Maximum Value 569.07 568.82 568.18 31c KpMinimum Value 567.81 568.19 567.72 [cm⁻¹] Δkp (2 mm□) 1.26 0.63 0.46Region Kp Maximum Value 568.99 569.01 568.4 31d Kp Minimum Value 567.71568.39 567.92 [cm⁻¹] Δkp (2 mm□) 1.28 0.62 0.48 Region Kp Maximum Value568.85 569.05 568.15 31e Kp Minimum Value 567.58 568.23 567.71 [cm⁻¹]Δkp (2 mm□) 1.27 0.82 0.44 Δkp (Entire Surface) [cm⁻¹] 2.89 1.5 0.93External Appearance after Breakage Crack Excellent Epitaxial Growth

TABLE 4 Experiment Experiment Experiment Example 9 Example 10 Example 11Oxygen Concentration Controlled Controlled Controlled Control SampleHolder Quartz SiC SiC Growth Substrate GaAs GaAs GaN Depression PatternDot Dot Dot ΔT [° C.] 6 3 3 T [° C.] 1000 1000 1000 GaCl PartialPressure [kPa] 3.06 3.06 3.06 NH₃ Partial Pressure [kPa] 6.12 6.12 6.12Formation of Formed Formed Not Formed Low-Temperature Buffer Layer

<Evaluation on GaN Substrates of Experiment Examples 9 to 11>

In the GaN substrate having the surface having a diameter of 100 mm, theexcellent result was obtained when Δkp (entire surface) was not morethan 2 cm⁻¹ (experiment examples 5 to 7); however, the GaN substratehaving the surface having a diameter of 150 mm was cracked even when Δkp(entire surface) was 1.5 cm⁻¹ (experiment example 10). The excellentresult was obtained, i.e., the GaN substrate was not cracked or brokenwhen Δkp (entire surface) was 0.93 cm⁻¹ (experiment example 11). Thus,the value of Δkp (entire surface), which serves as an index for absenceof crack and breakage, differs between a case where the diameter is 100mm and a case where the diameter is 150 mm, presumably because thermalstress during epitaxial growth of another semiconductor layer on the GaNsubstrate results from temperature distribution (temperature difference)in the GaN substrate. As the diameter of the surface of the GaNsubstrate becomes larger, it becomes much more difficult to reduce thetemperature difference in the surface of the GaN substrate in order toreduce thermal stress generated in the GaN substrate (generally, thermalstress generated in a GaN substrate is proportional to about the squareof the diameter of the surface of the GaN substrate. The GaN substratehaving a diameter of 150 mm has thermal stress twice or more than twiceas large as that of the GaN substrate having a diameter of 100 mm.).

Hence, the GaN substrate is less likely to be cracked during epitaxialgrowth of another semiconductor layer on the GaN substrate by decreasingthe residual strain of the GaN substrate as the diameter of the surfaceof the GaN substrate is larger. In order to reduce occurrence of crackand breakage during the epitaxial growth, it is considered preferablethat Δkp (entire surface) is not more than 1 cm⁻¹ in the GaN substratehaving a diameter of 150 mm.

Experiment Example 12

The threading dislocation density of a GaN substrate produced in thesame manner as in experiment example 5 was evaluated by way of etchpits. A solution of H₂SO₄:H₃PO₃=1:1 was heated at 250° C., the GaNsubstrate was immersed therein for about 30 minutes, and an etch pitdensity was measured using an optical microscope. The etch pits in thecentral portion of the GaN substrate were in a high density in thevicinity of the core, and were in a low density at a region away fromthe core. In a region with a radius of 50 μm from the core as itscenter, the etch pit density was not less than 1×10⁷ cm⁻² (pits werecontinuous without interruption). In a region with a radius of 400 μmtherefrom excluding the region with a radius of 50 μm from the core asits center, the etch pit density was 3×10⁵ cm⁻². A similar distributionof etch pit density was also obtained in GaN substrates produced in thesame manner as in experiment examples 6, 7, and 11, i.e., the etch pitdensity was not less than 1×10⁶ cm⁻² in a region with a radius of 50 μmfrom the core as its center, whereas the etch pit density was less than1×10⁶ cm⁻² in a region with a radius of 400 μm therefrom excluding theregion with a radius of 50 μm from the core as its center.

Experiment Example 13

An oxygen concentration distribution in a GaN substrate produced underthe same conditions as those in experiment example 5 was evaluated usingsecondary ion mass spectroscopy (SIMS). The oxygen concentration was2×10¹⁸ cm⁻³ in the facet growth region in the vicinity of the core, andthe oxygen concentration was 3×10¹⁶ cm⁻³ in the C plane growth regiondistant from the core (intersection of diagonal lines of 800 μm□ throughcores at four corners). Also, in a GaN substrate produced in the samemanner as in experiment examples 6, 7, and 11, the oxygen concentrationwas not less than 5×10¹⁷ cm⁻³ in the facet growth region and the oxygenconcentration was less than 5×10¹⁷ cm⁻³ in the C plane growth region.

Heretofore, the embodiments and experiment examples of the presentinvention have been illustrated, but it has been initially expected toappropriately combine configurations of the embodiments and experimentexamples.

The embodiments and experiment examples disclosed herein areillustrative and non-restrictive in any respect. The scope of thepresent invention is defined by the terms of the claims, rather than theembodiments and experiment examples described above, and is intended toinclude any modifications within the scope and meaning equivalent to theterms of the claims.

INDUSTRIAL APPLICABILITY

The GaN substrates of the embodiments and experiment examples can beused for semiconductor devices such as SBD.

REFERENCE SIGNS LIST

10: GaN substrate; 11: GaN crystal; 21: growth substrate; 21 a: surface;22: patterning layer; 23: threading dislocation; 24: supportingsubstrate; 25: bonded substrate; 30: orientation flat; 31 a, 31 b, 31 c,31 d, 31 e: region; 32: imaginary circle.

The invention claimed is:
 1. A gallium nitride substrate comprising asurface with a diameter of not less than 100 mm, a difference being notless than 0.1 cm⁻¹ and not more than 2 cm⁻¹ between maximum and minimumvalues of wave numbers at a maximum peak of peaks corresponding to an E₂^(H) phonon mode in micro-Raman scattering mapping measurement at eachof square regions having sides each having a length of 2 mm, the squareregions being located at a total of five locations including a centrallocation and four circumferential edge locations on the surface of thegallium nitride substrate, a difference being not less than 0.93 cm⁻¹and not more than 2 cm⁻¹ between maximum and minimum values of the wavenumbers at the maximum peak of the peaks corresponding to the E₂ ^(H)phonon mode at all of measurement points in the five locations whereineach of the square regions having the sides each having the length of 2mm on the surface of the gallium nitride substrate includes: a firstregion and a second region, an oxygen concentration of the first regionbeing higher than an oxygen concentration of the second region, thefirst region includes a core portion being a region having dislocationsintensively located therein, the second region being located away fromthe core portion.
 2. The gallium nitride substrate according to claim 1,wherein the diameter is not less than 150 mm, the difference is not lessthan 0.1 cm⁻¹ and not more than 1 cm⁻¹ between the maximum and minimumvalues of the wave numbers at the maximum peak of the peakscorresponding to the E₂ ^(H) phonon mode in the micro-Raman scatteringmapping measurement at each of the square regions having the sides eachhaving a length the 2 mm, the square regions being located at the totalof five locations including the central location and the fourcircumferential edge locations on the surface of the gallium nitridesubstrate, and the difference is not more than 1 cm⁻¹ between themaximum and minimum values of the wave numbers at the maximum peak ofthe peaks corresponding to the E₂ ^(H) phonon mode at all of themeasurement points in the five locations.
 3. The gallium nitridesubstrate according to claim 1, wherein the first region has a threadingdislocation density of not less than 1×10⁶ cm⁻²; and the second regionhas a threading dislocation density of less than 1×10⁶ cm⁻².
 4. A bondedsubstrate in which the gallium nitride substrate recited in claim 1 isbonded to a supporting substrate.
 5. The gallium nitride substrateaccording to claim 1, wherein the difference at each of the fivelocations is not less than 0.11 cm⁻¹ and not more than 1.79 cm⁻¹.
 6. Thegallium nitride substrate according to claim 1, wherein the differenceat all of the five locations is not more than 1.93 cm⁻¹.